The invention pertains to a method for powdering printed sheets, wherein the sheets are printed on one or both sides in a printing machine and powder is taken out of a supply container by means of a metering element and blown onto the sheets by means of an air current, and powder is always carried in the air current and ejected when a sheet is present. The invention also pertains to a device for powdering printing sheets, which contains a supply container for the powder and a metering element that is connected in series with the supply container and opens up into an air current that ejects the powder.
It is common practice to dust printed paper sheets with powder before stacking so that the still wet printing ink is not smeared. The sheets are usually dusted by a powdering device, in which powder is mixed with an air current and the powder is ejected in the air current and applied onto the sheet. The dusting devices usually contain a supply container that fills a powder container with powder. An air current is blown into this powder container such that the powder is whirled up and ejected with the emerging air. This air is subsequently blown onto the sheet. In dusting devices of this type, it has been determined that the quantity of the ejected powder highly depends on the quantity of powder situated in the powder container as well as the flow speed of the air current. It has also been determined that the type of powder represents a significant factor in this respect. A pulsed air current may also cause problems if the dusting frequency reaches the resonant range of the air current.
Consequently, the invention is based on the objective of providing a method and/or a device that can dust printing sheets in a more precise and significantly easier fashion while ensuring reduced powder loss.
According to this inventive method, powder is always introduced into the air current when a sheet surface to be dusted is present by timing the metering process such that it is coordinated with the presence of the sheet.
This means that an air current is always present, and that powder is always introduced into this permanent air current when there is a sheet surface to be dusted. In comparison with the state of the art, the method according to the invention provides the significant advantage that the admixing of powder is timed instead of the air current. Consequently, resonance that could negatively influence the transport of the powder or the printed paper web cannot form in the air current.
Since the powdering device is not situated directly at the outlet of the air current, i.e., it is situated at a distance from the paper web, the powder is added in a metered fashion at a certain advance time, with this advance time being synchronized with the flow speed of the air current and the length of the air conduits. The powder is ejected precisely when a printed paper sheet is situated underneath the blow nozzles. Air that does not contain any powder is ejected if no paper sheet is situated underneath the blow nozzles.
According to one additional development of the invention, the air current blows through the metering element, i.e., the powder is removed from the metering element by the air current. The advantages of this method can be seen in the fact that the powder is already mixed with air and that the method can be carried out without creating any wear.
The objective of the invention mentioned above is attained with a device that can drive the metering element in a cyclical fashion, with the timing corresponding to the presence of a sheet to be powdered. This metering element always admixes powder with the air current when a printed sheet needs to be dusted. This provides the significant advantage that, as mentioned previously, a permanent air current is present and a pulsating or oscillating air column can be prevented. Powder is only introduced into this air current when necessary, i.e., when a sheet is transported past the dusting device.
In the embodiments of the invention, the metering element contains a rotationally symmetrical body, e.g., a cone, a ball, a roller, in particular, with a horizontally driven rotational axis or a metering band. These transport devices transport the powder from the supply container into the air current, with the transport elements being driven in a cyclical fashion.
In one embodiment, in which the metering element is realized in the form of a roller, the roller has an uneven surface viewed in the transport direction containing grooves that extend in the circumferential direction and are arranged on the periphery of the roller. These grooves provide the significant advantage that the surface of the roller receiving the powder is significantly increased, whereby the adhesive forces that hold the powder on the roller are substantially higher than in metering elements with a smooth surface. The powder actually adheres to the grooved roller so well that it does not fall out of the grooves due to gravity.
A roller that is provided with cells on its surface is known from U.S. Pat. No. 4,867,063. Although this cell conveyor also makes it possible to easily transport pourable material, the cells gradually become clogged when powder is transported, so that the volume being transported decreases over time. This means that a constant volume transport over an extended period of time cannot be ensured with cell conveyors of this type.
According to an additional configuration of the invention, the metering element is provided with a device for removing the powder from the metering element. This device consists of a blade, a brush, a blow nozzle or the like. Devices of this type are able to project into the circumferentially extending grooves of the roller relatively easily and remove the powder in this fashion. This is not possible with cell conveyors because, for example, a blade is unable to project into individual cells.
According to a refinement, the metering element has a powder-friendly surface.
In another embodiment, the metering element is realized in the form of a circular disk with concentric rings, and the opening of the supply container is located in one section such that powder is transported away from the region of this opening by the concentric rings. The powder is subsequently lifted out of the concentric rings by a blade. The powder could also be blown out of the rings with a suitable device containing blow nozzles.
The cyclical operation of the metering element is achieved by using a step motor as the drive. This step motor has a variable rotational speed and frequency and is controlled by the transport device (sheet feeder) for the printed paper sheets. A change in the driving speed of the step motor causes a change in the volume flow, and the metering element can be adapted to the size and speed of the paper sheets by varying the cycle frequency.
In order to prevent the powder from flowing out of the supply container, the metering element contains an outlet opening that is offset relative to the inlet opening by more than 180xc2x0 in the rotating direction. Due to this measure, the powder is prevented from flowing directly from the inlet opening to the outlet opening through the grooves in the roller of the metering element and is admixed with the air current in an uncontrolled fashion.